(1) Field of the Invention
The present invention is directed to the reduction or elimination of undesirable shunt currents in electrochemical cell devices having a plurality of cells connected, at least in part, in series, and having an electrolyte which is a common electrolyte to at least two of these cells, and which includes shared electrolyte, whereby an electrical electrolytic conductive bypass path is created around these cells and through said shared electrolyte, which results in undesirable shunt currents. More specifically, the present invention is directed to such shunt current reduction or elimination by appropriate application of a protective current, and to an electrochemical device for achieving this result.
(2) Prior Art
In multicell electrochemical devices having a plurality of cells in series and having a common electrolyte, e.g. circulating through the cells, shunt current losses (also known as current bypasses) occur as a result of conductive paths through the electrolyte during both charge and discharge. These shunt current losses may also occur under open circuit conditions, and cause undesired discharge of electrochemical devices. Additionally, these shunt currents may have secondary undesirable effects on electrochemical devices. For example, uneven or improper plating of a functional component may occur, ultimately resulting in a shortened utility of the device. Also, corrosion of the electrodes and/or other components may occur, reactants may unnecessarily be consumed and excess thermal losses may result. Thus, shunt current problems have been recognized in the field of electrochemical devices for many reasons, and various modifications to such devices have been made to reduce or eliminate these as well as other recognized problems.
For example, it has been suggested that multiple cell systems include electrical isolation means for minimizing shunt current effects. Thus, U.S. Pat. No. 3,773,561 (Bjorkman) teaches that internal short circuiting of a plurality of electric cells of a cell stack may be prevented during shutdown, or standby, by sealing off the cells from electrical contact with each other by closing off inlet and outlet ports to isolate electrolyte portions in the individual cells. U.S. Pat. No. 3,806,370 (Nischik) describes an electrolyte interrupter system for providing intermittent flushing of the electrolyte in a fuel cell battery having several fuel cells in which the electrodes are held in plastic frames. The electrolyte interrupter system is made up of an electrolyte distributor and an electrolyte manifold arranged in the frames of the individual fuel cells. Electrolyte supply ducts for each cell open into the electrolyte distributor, and electrolyte discharge ducts for each cell open into the electrolyte manifold. The electrolyte distributor and the electrolyte manifold are each formed by mutually aligned holes in the upper portions of the frames, with the bottom of the holes forming the electrolyte distributor being located at least at the same height as the openings of the electrolyte discharge ducts leading into the electrolyte manifold. U.S. Pat. No. 3,378,405 (Schumacher et al) teaches the electrical isolation of cells from one another in a sodium amalgam anode-oxidant multicell fuel cell system by using one, and pref,erably two, dielectric interrupters per cell. U.S. Pat. No. 4,025,697 (Hart) describes multicell devices in which electrolyte is distributed in a two stage system in which a large pump (first stage) distributes the electrolyte through hydraulically driven circulators (second stage) to individual electrode compartments which are electrically isolated from each other. The overall system results in minimizing intercell leakage and intercell power losses through shorting circuits through the electrolyte.
Other techniques for electrolyte interruption, as a means for preventing internal or shunt current losses in multicell devices, have also been taught. For example, U.S. Pat. Nos. 3,537,904 (Matsuda et al) and 3,522,098 (Sturm et al) describe the insertion of gas bubbles into the electrolyte solution to reduce or break up the conductive path through the electrolyte.
Alternative methods have also been suggested. For example, U.S. Pat. No. 3,666,561 (Chiku) describes an invention which provides an electrolyte circulating battery in which the flow of current between cells is minimized by having branded electrolyte inlet and outlet passages to and from the cells, these passages being greatly lengthened and considerably reduced in cross-section so that the electrical resistance of the electrolyte in each branched passage is increased. The patent also teaches further preventing internal currents by the use of gas bubbles injected into the electrolyte paths to further increase electrical resistance.
Geometric redesign has also been employed without gas bubbles to prevent or reduce shunt current or internal circuit losses. For example, U.S. Pat. No. 3,964,929 (Grevstad) teaches shunt current protection in fuel cell cooling systems by providing coolant circulation means and plenums adapted to create high electrical resistance paths. U.S. Pat. No. 3,540,934 (Boeke) points out that in-series multicell redox systems may have shunt current problems even when electrically non-conductive tubing is used. The patent teaches that electrical shunting will cause negligible inefficiency if the individual electrolyte fluid passages, connecting each individual electrode chamber with a central flow system, have a length to average inside diameter ratio of ten to one or more. U.S. Pat. No. 3,634,139 sets forth a design approach to the shunt current problems. The patent teaches that leakage currents can be minimized by proper manifold design. As an example, it is stated that by making electrolyte branch (or channel) ports small even though the manifold diameter is relatively large, leakage current can be neglected. However, if the ports are made too small, electrolyte flow may be retarded. The patent states that ports of about one-tenth of an inch in diameter are acceptable and manifolds of about one-eighth of an inch in diameter are acceptable.
U.S. Pat. No. 4,049,878 (Lindstrom) is representative of the present state of the art effort to solve leakage current problems. This patent indicates that many electrochemical devices contain a plurality of cells in stacked formation, which cells may be coupled in parallel groups, which groups are in turn coupled in series. Other embodiments are multicell devices in which the cells are only coupled in series. It is stated that more complicated coupling patterns are possible which are determined by the desire to reduce leakage currents in the electrolyte system and to create conditions for special electrical control modes with in-and-out coupling of individual parts of the stack. It is also pointed out that the natural way to reduce leakage currents is to minimize the dimensions of electrolyte channels, but that this technique results in electrolyte flow problems. The patent teaches a manner in which these problems may be avoided. The technique involves the use of fluid connections or cross-channels which are set up between the electrolyte spaces in the cells, which cells are being coupled electrically in parallel. These cross-channels are, in one embodiment, arranged in the lower parts of the electrolyte spaces so that some electrolyte is transferred between these electrolyte spaces by means of the cross-channels. In another embodiment, the cross-channels are also provided between the electrolyte spaces in the parallel-connected cells in the upper parts of the electrolyte spaces in order to produce a so-called plenum.
In a recent article by Burnett and Danley, of Monsonto "Current Bypass in Electrochemical Cell Assemblies", presented at the American Institute of Chemical Engineers' National Meeting, Atlanta (Feb. 26-Mar. 1, 1978) Symposium on Electro-organic Synthesis Technology, Session 1, Operating Experience with Electro-organic Processes, the problems of shunt current in circulating electrolyte multicell in-series devices is examined and derivations of certain mathematical relationships between geometry related currents and resistances in such devices are developed. The authors conclude that current bypass losses for certain cell arrangements may be held at an acceptable level, but that the losses increase rapidly with an increasing number of cells. Further, no specific solution for elimination of shunt current or current bypass of the type used in the present invention is derived or suggested. In fact, the authors describe 8 ft. long cell connections to the manifold to reduce the losses effected by shunt currents.
Recently issued U.S. Pat. No. 4,081,585 (Jacquelin) appears to be the only prior art reference which reduces leakage currents by nulling with electrodes. However, unlike the method and device of the present invention, this patent teaches the use of at least four times as many sets of electrodes as modules of cells and employs these electrodes in branch channels, an inferior and expensive technique at best.
Notwithstanding all of the foregoing efforts in the field to ovecome shunt current (leakage current) problems in multicell electrochemical devices, the novel and effective technique of the present invention has not heretofore been taught or suggested. In fact, the considerable prior art teachings as represented by the above references are directed toward problematic techniques which themselves create design and flow difficulties. These prior art teachings are in a direction away from the present invention and support the patentability thereof.